Acid-base titration

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Titration setup. The burette would normally be held by a clamp, not shown here. The pink is most likely caused by use of the phenolphthalein indicator.
Titration setup. The burette would normally be held by a clamp, not shown here. The pink is most likely caused by use of the phenolphthalein indicator.

An acid-base titration is a method in chemistry that allows quantitative analysis of the concentration of an unknown acid or base solution. It makes use of the neutralization reaction that occurs between acids and bases, and that we know how acids and bases will react if we know their formula.

[edit] Equipment

The key equipment used in a titration are:

  • Buret
  • White Tile - used to see a colour change in the solution
  • Pipette
  • Acid/Base Indicator (the one used varies depending on the reactants)
  • Erlenmeyer flask
  • Standard Solution (a solution of known concentration, a common one is aqueous Na2CO3)
  • Solution of unknown concentration
  • It is also recommended that a control is used in order to see the difference in colour between the initial and final colour.

[edit] Method

Before starting the titration a suitable pH indicator must be chosen. The endpoint of the reaction, when all the products have reacted, will have a pH dependent on the relative strengths of the acids and bases. The pH of the endpoint can be roughly determined using the following rules:

  • A strong acid reacts with a strong base to form a neutral (pH=7) solution.
  • A strong acid reacts with a weak base to form an acidic (pH<7) solution.
  • A weak acid reacts with a strong base to form a basic (pH>7) solution.

When a weak acid reacts with a weak base, the endpoint solution will be basic if the base is stronger and acidic if the acid is stronger. If both are of equal strength, then the endpoint pH will be neutral. However weak acids are not often titrated against weak bases because the colour change shown with the indicator is often not rapid enough.

A suitable indicator should be chosen, that will experience a change in color close to the end point of the reaction.

First, the buret should be rinsed with the standard solution, the pipette with the unknown solution, and the conical flask with distilled water.

Secondly, a known volume of the unknown concentration solution should be taken with the pipette and placed into the conical flask, along with a small amount of the indicator chosen. The buret should always be filled to the top of its scale with the known solution for ease of reading.

The known solution should then be allowed out of the buret, into the conical flask. At this stage we want a rough estimate of the amount of this solution it took to neutralize the unknown solution. Let the solution out of the buret until the indicator changes color and then record the value on the buret. This is the first titre and should be discluded from any calculations.

Perform three more titrations, this time more accurately, taking into account we know roughly where the end point will occur. Take note of each of the readings on the buret at the end point, and average these at the end. Endpoint is reached when the indicator just changes color permanently. This is best achieved by washing a hanging drop from the tip of the buret into the flask right at the end of the titration to achieve a drop that is smaller in volume than what can usually be achieved by just dripping titre off the buret.

Acid-base titration is usually performed with a phenolphthalein indicator.